In prior copending application for patent, Ser. No. 6,277, filed Jan. 25, 1979, now U.S. Pat. No. 4,239,396, disclosed a high capacity, truck-mounted blender in which a high speed impeller is mounted for rotation concentrically within an outer casing and has a solids inlet which is isolated from an outer concentric liquid inlet. In the preferred form of that invention, the blender is specifically designed for use in cementing operations or in fracturing oil and gas subsurface formations. The high speed impeller is positioned in inner spaced concentric relation to an annular chamber which causes the liquid to be directed axially past the discharge side of the impeller whereby solid material introduced through the central inlet is discharged by the impeller under centrifugal force into the fast-moving, axial stream of liquid. A mixing chamber diverges in an axial direction away from the impeller zone into a discharge port. Further, a recirculation inlet is provided to establish communication from the discharge side and the central or solids inlet so as to permit any excess of the blended material to be recirculated through the blender. A number of important advantages are seen to accrue from the isolation of the solids inlet from the liquid inlet, particularly at the interface across the impeller zone. Further, it has been found possible to greatly improve the blending of at least certain materials by closely controlling the movement of the liquid stream into the blender and along the annular space formed in surrounding relation to the impeller zone. Furthermore, it has been found that the versatility of the blender apparatus can be greatly enhanced by the use in combination therewith of a closed loop system which along with the blender can be vehicle-mounted and operated off the vehicle drive to regulate the delivery of materials to and from the blender as well as to regulate the discharge of blended materials from either side of the vehicle into a well head or other intended site of use.